Cured silicone resins show excellent heat resistance, weather resistance, oil resistance, low-temperature resistance, and electric insulation and, at the same time, low modulus of elasticity and low stress. In consequence, cured silicone resins are used in a wide variety of applications including protection of electronic parts installed in vehicles and electrical appliances. In recent years, there is a demand especially for cured silicone resins which are flexible and tough.
In order to obtain cured silicone resins, curable reactive functional groups are generally introduced to silicone resins. In particular, in order to obtain cured products which are flexible and tough, silicone resins are synthesized by utilizing such means as introduction of long silicone chains and optimization of the number of functional groups or produced in the form of polymer blends by mixing resins.
The processes such as the following have been used for the synthesis of silicone resins up to the present: condensation of silanols; linking of readily hydrolyzable groups such as aminoxy groups, alkoxy groups, and oxime groups to the siloxane chain followed by polycondensation in the presence of atmospheric moisture; condensation reaction with the use of chlorosilanes and hydrosilylation reaction between a compound in which a hydrogen atom is linked to a silicon atom and a compound in which an unsaturated aliphatic hydrocarbon group such as a vinyl group and an ally group is linked to a silicon atom in the presence of a Group 8 compound as a catalyst.
However, it is difficult to introduce curable reactive groups selectively according to the aforementioned processes based on polycondensation. Further, it is a matter of concern that the resins thereby obtained are apt to increase in viscosity or to gel and their storage stability is questionable. Now, addition of tertiary amines is proposed as a measure to suppress increase in viscosity and gelation of the resins prepared by the hydrosilylation reaction; however, even this measure could not fully suppress increase in viscosity and gelation of the silicone resins during the course of purification where heat is applied under reduced pressure (refer to patent document 1).
An example of a silicone resin containing curable reactive functional groups is afforded by a silicone resin in which radically reactive methacryl groups are introduced to the silicone chain; however, a resin containing methacryl groups at both ends undergoes crosslinking at fewer places to yield a product which tends to break easily (refer to patent document 2).
When the technique of polymer blending is used, the compatibility of polymers to be blended poses a problem. When the polymers do not melt into one another, the resulting blend becomes turbid and it cannot be used as an optical material. Blends of polymers whose monomers are mainly composed of carbon-chain compounds face a problem that the cured products turn yellow at high temperatures. Hence, a demand has been created for development of silicone resins in whose silicone chains are selectively introduced arbitrarily long silicone chains and an arbitrary number and kind of curable reactive groups so that the resins can yield cured products with excellent flexibility, toughness, heat resistance, and transparency.
Materials of low birefringence, low optical modulus of elasticity, and high optical transparency are used as optical materials in such applications as bases of liquid crystal display devices, optical lenses, and encapsulating materials for light emitting diodes. Moreover, the manufacturing process necessitates that the materials intended for use in bases of liquid crystal display devices and optical lenses have high heat resistance. Glass has been used as a material that satisfies the aforementioned requirements.
However, curved surfaces are used in optical lenses and, in recent years, a demand is growing for increasingly thinner bases for liquid crystal display devices. Glass hitherto used in these applications has a problem in strength because of its brittleness and this property has begun to limit the area of applications of glass.
Polymers are considered tough materials, but they generally show low heat resistance. As a measure to provide polymers with high heat resistance, introduction of an aromatic skeleton is under study, for example, in the case of thermoplastic resins. However, this measure leads to an increase in birefringence and optical modulus of elasticity on the other hand and it is difficult for a given thermoplastic resin to show both high heat resistance and good optical properties simultaneously. Furthermore, in the case of thermosetting resins, those known thus far are reported to become colored during heat curing and they are not suitable for use as optical materials. For example, acrylic resins have a property of rapid curing, but they occasionally become colored on heating because of their low heat resistance.
Silicone resins are generally known as materials of excellent heat resistance and high transparency and they are useful as highly flexible materials as well. However, the curable silicone resins known thus far require high temperature and long time in curing and have the disadvantage of low productivity. For example, in a document which describes curing by hydrosilylation of silicone resins, a specimen cures at a high temperature of 60° C. in 1 hour or at room temperature in a long time of 24 hours (refer to patent document 3).    Patent document 1: JP Hei 4-352793 A    Patent document 2: JP2002-302664 A    Patent document 3: JP2007-126576 A